1. Field of the Invention
The invention relates to a structure of a concentrated winding coil provided on a stator of a rotary electric machine.
2. Description of Related Art
In order to reduce the loss of an electric motor, it is effective to increase the coil space factor (conductor's cross-sectional area/coil's cross-sectional area) of the stator. Therefore, a structure in which thick flat rectangular wires whose cross-sectional shape is rectangular, as conducting wire for forming coils, are wound around the teeth so that each tooth has thereon a single layer of winding is more often employed than a structure in which round wires whose cross-sectional shape is round are wound around the teeth so that each tooth has thereon a plurality of layers of winding. In the meantime, a construction of a stator in which each of the teeth extends radially inward and the width of each tooth in the circumferential direction (the circumferential width of each tooth) is substantially constant in the radial direction is increasingly employed. In this stator construction, the circumferential width of the slots between the teeth in which coils are inserted becomes gradually narrower toward the side of the top ends of the teeth (the inner peripheral side of the stator). In the case where a thick flat rectangular wire is wound around a tooth so as to form a coil of a single layer of winding, it is difficult to change the cross-sectional shape of the coil in the lengthwise direction of the tooth, which gives rise to a problem of the clearance between the coils of adjacent teeth being unfavorably small at the inner peripheral side of the stator, unlike the case where a round wire is wound around a tooth so as to form a coil of a plurality of layers of winding.
Therefore, a method in which the cross-sectional shape of a winding wire is changed so that the cross-section of an entire coil on each tooth is a trapezoid whose height becomes lower toward the top end of the tooth (the inner peripheral side of the stator) according to the shape of the slots between the teeth has been proposed (see, e.g., Japanese Patent Application Publication No. 2001-178051 (JP 2001-178051 A)).
However, the coil described in JP 2001-178051 A requires that the cross-sectional shape and the cross-sectional area of conducting wires that form the coil be changed every turn of the winding, giving rise to a problem of complication of the shape of the winding wire and therefore an increase in cost.